1. Field of the Invention
The present invention generally relates to the production of laminates formed of a substrate and a coating, and more particularly to making laminated self-supporting structures such as panels and the like stratiform elements of the type used at the interior of automobiles, e.g. as inner door panels.
2. Description of the Prior Art
Large stratiform and generally shaped moldings based upon synthetic polymers are generally produced by first molding the base substrate portion of the laminate in a molding device, e.g. a molding press, and by subsequently connecting the molded and generally rigid substrate with a suitable coating layer, e.g. by adhesive techniques or by mold pressing.
When trying to simplify this technique by using a laminated starting material which is molded in a press or the like device for producing a generally stratiform and shaped (profiled) self-supporting product, a general problem is the edge portion of the molded laminate.
The interior door panel of an automobile, for example, must have a substantially rigid supporting stratum which is coated on its passenger-exposed side with a flexible material for reasons of appearance and safety (cushioning effect). Now, assuming that an unshaped preformed laminate were to be used and shaped in a molding device, e.g. a conventional molding press, it is easily understandable that the support stratum will be exposed, that is, not covered by the coating layers, in its edge portions. While some parts of the resulting edge portions might be covered, when installed in an automobile door, by supports in the door, other edge portions will remain exposed to the eye and, what is even more critical, constitute a potential hazard.
For this reason, conventional automobile door panels are made by first shaping or molding the substrate portion and subsequently applying the coating or cushioning layer; the cushioning layer is dimensioned such that it overlaps the substrate, and its overlapping edge portions can be folded around any exposed edges of the substrate.
No conventional method known by applicant for producing self-supporting laminated structures is capable of avoiding the edge-covering problem set forth above in a one-stroke operation.
For example, U.S. Pat. No. 2,797,179 discloses a method for producing molded laminates in a molding press; in a first stroke of the press a coating layer is formed and shaped; then, the press is opened again to remove, reverse and reinsert the coating layer. In a second stroke of the press, the substrate is formed, laminated with the coating layer and cut by a shearing edge of the lower mold member or plate. At the cut edge of the laminated product obtained the substrate is exposed, that is, not covered by the coating layer. As is readily apparent, such operation is a two-stroke method (each stroke including a closing/opening cycle of the press) and thus rather time consuming per se. Further, no way to resolve the cut edge problems is apparent from the disclosure of U.S. Pat. No. 2,797,179 which relies on thermosetting materials for the substrate of the laminates.
Shaping (molding) of thermoplastic sheet materials per se is well known to the art, cf. U.S. Pat. No. 3,007,201. This technology is based upon thermal plastification, that is, heating an inherently rigid or brittle material to a temperature where such material becomes moldable, e.g. by stretching or drawing. It is believed by applicant that the concept of "thermal plastification" of thermoplastic polymers or polymer-based compositions includes a variety of transformations, depending upon the polymer and/or the filler-type constituents; the term as used in the present invention includes, for example, the temperature range of elastic elongation specified in U.S. Pat. No. 3,007,201 as well as plastification by softening of the polymer constituent when the latter is heated to a temperature above its softening or even melting temperature. For example, a suitable material for use as substrate in the present invention is disclosed in British Patent Specification No. 1,457,015 and comprises a thermoplastic polymer, such as polypropylene, and a non-thermoplastic filler, such as saw dust; such a composition is "thermoplastic" in the sense used herein and can be thermally plastified (used interchangably herein with plasticized) by heating to temperatures well above the melting point of the polymer constituent.
Turning back to the art of shaping thermoplastic sheet materials, such as by the methods disclosed in the above mentioned U.S. Pat. No. 3,007,201 or in German Published Patent Application DE-AS No. 1,158,697, such methods include no lamination steps. Of course, various coating techniques are known in the thermoplastics art, e.g. from German Published Patent Application DE-OS No. 2,448,361, but here again the substrate is molded first and the coating layer applied thereafter, and, again, the above explained problem of the uncoated substrate edge is not resolved.